Glass melting furnace

ABSTRACT

The present invention relates to a glass meting furnace comprising a channel-shaped melting tank, the batch materials being introduced at an upstream end, the molten glass being recovered at the downstream end, said furnace being heated by means of burners, in which the combustion energy is produced by oxy-fuel combustion in respect of at least 65% thereof, the burners being distributed on the walls along the length of the furnace, in which flue gas discharge is mostly localized close to the upstream end near the openings through which the batch materials are introduced, the rest of the flue gas being removed close to the downstream part so as to maintain dynamic sealing with respect to the surrounding atmosphere.

This application is a Divisional of U.S. Ser. No. 12/934,412 filed Sep.24, 2010, pending, which is a 371 application of PCT.EP09/53518 filedMar. 25, 2009 and claims the benefit of EP 08102882.1 filed Mar. 25,2008. The contents of each of these applications are incorporated hereinby reference in their entirety.

The present invention relates to glass melting furnaces in which themelting energy is produced essentially by burners fed with fuel and withoxygen or a gas very rich in oxygen. These furnaces are usually termed“oxy-fuel combustion” furnaces.

The auxiliary use of oxy-fuel combustion burners is well known in glassmelting furnaces. On furnaces operating conventionally with air, anoxy-fuel combustion burner or a limited number thereof are then added.The purpose of introducing these additional burners is generally toincrease the capacity of existing furnaces, possibly when theirperformance is seen to decline owing to their old age. This situation isfound for example when the regenerators associated with such furnaceshave become degraded and are no longer able to heat the air used forcombustion sufficiently. The capacity of a given furnace may also simplybe increased by introducing additional energy sources.

As a general rule, the additional oxy-fuel combustion burners are placedclose to the zone in which the batch materials are charged into thefurnace. These burners thus melt the batch materials. The addition of afew oxy-fuel burners in large-capacity furnaces is usually accomplishedwithout any substantial modification to the general operation of thefurnace in the sense that, in particular, the regenerators continue tooperate and therefore handle both the combustion flue gas arising fromburners operating with air and that arising from burners operating withoxygen.

Beside the fact of having an additional energy source, these systemsoperating in what is called “oxy-boosting” mode do not provide thebenefit of all the known advantages that may result from oxy-fuelcombustion. Among the number of potential advantages are mainly a lowerenergy consumption and reduced emissions of undesirable flue gas.

Oxy-fuel combustion provides an energy saving at least for the reasonthat the energy of combustion gas is not partly absorbed by the nitrogenof the air. In conventional furnaces, even if some of the energy carriedoff with the nitrogen is recovered in the regenerators, the flue gasfinally discharged still gives off a significant amount of energy. Thepresence of nitrogen contributes to this loss.

Reducing the energy consumption by a production unit in question has inaddition the advantage of consequently limiting the carbon dioxideemissions and therefore of meeting the statutory requirements in thisfield.

The presence of nitrogen is also a source for the formation of nitrogenoxides, called NOx, the emission of which is practically prohibitedbecause of damage due to the presence of these compounds in theatmosphere. In practice, users strive to operate furnaces underconditions leading to emissions that are as low as possible. In the caseof glass furnaces, these practices are not sufficient to meet the verystringent standards in force, and it is necessary to carry out anexpensive flue gas decontamination operation by the use of catalysts.

By using oxygen, it is possible to circumvent the problems associatedwith the nitrogen in the air, something which is not the case inoxy-boosting techniques.

Despite the abovementioned advantages, the use of oxy-fuel combustion inlarge glass furnaces remains to be developed. The reasons for this areof several types. Firstly, the use of oxygen is necessarily moreexpensive than that of air.

The economic assessment of the use of oxy-fuel combustion is positiveonly if it is possible to recover a significant amount of heat from theflue gas. Hitherto, recovery of this energy does not seem to have beenaccomplished satisfactorily and the potential energy saving has not beenactually achieved.

Moreover, the implementation of oxy-fuel combustion still posestechnical problems that counteract certain advantages. One recognizeddifficulty is due to the corrosion of the refractories, this corrosionreducing the lifetime of the silica refractories of the furnace roof.This is because the high H₂O content of the combustion atmosphere causestwo deteriorating effects:

-   -   the first is due to the diffusion of H₂O into the glassy phase        of the refractory blocks; and    -   the second is due to the condensation of sodium hydroxide        present in the atmosphere on the refractory bricks, which        entails a high degree of oxidation, in fact six times higher in        the case of an oxy-fuel combustion furnace.

To obviate these conditions, it is necessary to use materials that aremore corrosion-resistant than those normally chosen. Usually, forvarious reasons, the roof of large glass furnaces is made of silicabricks. In the case of an oxy-fuel combustion furnace, it is necessaryinstead to use materials such as alumina, AZS or spinels. However, thesematerials are more expensive and also pose problems because they aresignificantly heavier.

Further, new problems have also appeared in practice that requirespecific new operating conditions to ensure that this technique beeffectively used in applications that the theory suggests would beadvantageous. The invention relates to ways of implementing the oxy-fuelcombustion technique in large glass furnaces that form the subjectmatter of the claims appended to the present description.

The inventors addressed the problem of the economics of this oxy-fuelcombustion technique. In particular, they provide a way of ensuring thatthe furnace flue gas energy is largely recovered and used for preheatingthe oxygen and, where appropriate, the fuels consumed. Some of the fluegas heat may also be used for preheating the batch materials chargedinto the furnace.

The economics, especially as regards energy, requires the flue gas heatto be recovered. The principle is known but the difficulty stems fromemploying recovery techniques for operating the furnace itself.

The inventors have chosen to utilize the flue gas energy, especially topreheat the oxygen. For obvious reasons, the use of regenerators forthis recovery is excluded. The operation has to be carried out inspecific heat exchangers, which operation is not very easy because hotoxygen is extremely corrosive for all materials with which it is incontact. This corrosive character is all the more pronounced the higherthe temperature reached by the oxygen.

It is also necessary according to the invention for the furnaces inquestion to be substantially free of a nitrogen-containing atmosphere.For this reason, unlike certain solutions proposed previously, it ispreferable to ensure that all the burners of the furnace operate inoxy-fuel combustion mode. Although nevertheless it is possible tomaintain part of the combustion in air-fuel combustion mode, the energygenerated by oxy-fuel combustion represents at least 65%, preferably atleast 80% and even more preferably at least 90% of the total energyconsumed in the furnace.

The use of an air-fuel combustion fraction may stem from a limitednumber of burners operating entirely in air-fuel combustion mode, but itmay also stem from the use of oxygen having a certain content of air. Inthe latter case, because the burners used in oxy-fuel combustion modehave particular characteristics, the oxygen/air mixture must have anoxygen content of at least 80% and preferably at least 90%.

To simplify matters in the rest of the description, reference is madeboth to oxy-fuel combustion and to “oxygen” combustion. Developments inthis subject, unless otherwise indicated, cover the use of oxy-fuelcombustion with an oxygen that may contain a small amount of air, or anassembly comprising a limited part operating in air-fuel combustion modecombined with predominantly oxy-fuel combustion.

Independently of the constituents of the furnace atmosphere resultingfrom the combustion, it is also necessary to prevent as far as possibleany penetration of air coming from the outside, on the one hand to avoidan energy loss corresponding to heating this air, but most importantlyto prevent as far as possible undesirable NOx formation due to this airreaching the high temperatures of the combustion flame (thesetemperatures are around 1800 to 2300° C. depending on the type of oxygenburner chosen).

Irrespective of the construction envisaged, glass furnaces cannot bekept completely impermeable to the external atmosphere. The effortsreported for this purpose have related mainly to the installation ofphysical barriers that limit the flow of gas from the outside into thefurnace. These measures are certainly useful, but appear to beinsufficient if it is desired to maintain an atmosphere essentiallyformed by the combustion gases.

According to the invention the intake of the surrounding atmosphere isconsequently prevented by arranging the furnace in such a way thatdynamic sealing develops. To do this, according to the invention, theflow of flue gas in the furnace has to be regulated in the mannerexplained in detail below.

In large glass melting furnaces, especially those using regenerators,gases flow in the furnace transversely. The burners are distributed oneither side of the tank containing the molten glass and they operatealternately. During one period, all the burners located on one side ofthe furnace are activated and the corresponding flue gas is dischargedvia the ducts located on the wall that faces them. The flue gas ispassed over the regenerators corresponding to the side in question.During the next period, it is the burners on the other side that are inaction, with the air flowing over the preheated regenerators, and so on.

In the case of oxy-fuel combustion furnaces, the burners located oneither side of the furnace operate continuously. The distribution of theburners on both sides is not determined by the need for thisalternation, because it does not exist, but more because of the desireto optimize the heat exchange between the flames and the glass melt orbetween the flames and the supernatant batch materials.

The flames of oxy-fuel burners are, for the same power, shorter than theflames of air-fuel burners. The reason for this is in particular becausethe gas flow is less voluminous because of the absence of nitrogen. Inorder for the energy distribution to be as uniform as possible, for asimilar furnace width, it is therefore desirable to place the burners onboth sides so that the surface of the melt is best covered.

Any increase in the emission velocity of the oxygen-fuel combustiongases, which could lengthen the flame, are not desired, in particular soas not to promote dust fly-off.

It is also preferable to ensure that the flames develop in as littledisturbed a manner as possible. To prevent collision between oppositelypositioned flames, the burners are therefore advantageously staggered.

According to another specific feature of oxy-fuel combustion flames, toachieve combustion staging over the length of the flame, which ispreferable, as in air-fuel combustion burners, it is advantageous toensure that the flames from these burners develops as a sheet lying in aplane substantially parallel to the surface of the glass melt. This isobtained for example by means of burners having a plurality of oxygeninjection nozzles located on either side of the fuel intake nozzle, allthese nozzles being substantially aligned so as to be parallel with thesurface of the melt.

The flue gas from the flames does not flow transversely, as in air-fuelcombustion. The flow is organized according to two objectives.

Firstly, it is necessary to ensure that heat transfer from the flue gasto the glass melt is as great as possible. In other words, efforts aremade to ensure that the flue gas temperature at the outlet of thefurnace is as low as possible, also taking into account the fact thatoxy-fuel combustion flames are at a higher temperature and that overallthe flue gas is also at a higher temperature than in air-fuel combustionmode.

To achieve greater heat exchange, the residence time in the furnace isextended.

Owing to the fact that, for the same amount of energy dissipated, theflue gas volume is reduced by more than a half compared with that inair-fuel combustion, for a furnace of identical volume, all other thingsbeing equal the flue gas residence time would necessarily be extended.

Secondly, the provisions relating to flue gas flow also help to improvethe heat transfer with the melt. In particular, this results from thepositioning of the flue gas outlets, the location of the burners and thedistribution of the power developed locally by each of these burners.

According to the invention, and to achieve better energy transfer withthe melt and with the batch materials, it is necessary to make the fluegas, or at least the major portion thereof, flow in the oppositedirection to the flow of the melt. Thus, the temperature of the flue gasas it progresses through the furnace decreases up to the point where itis discharged from the furnace.

For this reason, the discharge of the flue gas, or at least the majorportion thereof, is localized close to the point where the batchmaterials are charged into the furnace. One option is to ensure that theflue gas is discharged via ducts that are separate from those via whichthe batch materials are fed into the furnace. Another option is for thisdischarge to take place via the charging channels themselves, andtherefore countercurrently with the batch materials. In the latteroption, it is necessary in particular to avoid the risk of anyagglomeration due to the water vapour contained in the flue gascondensing upon contact with these “cold” batch materials.

To have the best heat transfer, most of the flue gas is discharged at apoint close to where the batch materials are charged into the furnace.In practice, this means at least 65%, and preferably at least 75%, ofthe flue gas.

The excess flue gas, that is not discharged as indicated above, followsa path intended in particular for maintaining dynamic sealing withrespect to the external atmosphere. At least part of this excess isadvantageously discharged towards the downstream end of the furnace. Asindicated, this fraction of the flue gas is as small as possible,advantageously being less than 35%, and preferably less than 25%, of allof the flue gas.

The downstream discharge of the flue gas takes place beyond the lastburners. It is necessary to prevent the combustion gases from beingdischarged before as complete as possible heat transfer has taken place.To do this, these gases must remain for some time in the furnace, hencethe need not to place the burners too close to the discharge ducts.

The presence of outlets downstream of the burner zone makes it possiblein particular to prevent air coming from this zone passing through theburner zone, since most of the air downstream comes from theconditioning zone. The NOx content is systematically detected at theupstream outlet. If the NOx content proves to be too high, it ispossible to correct this content according to the invention byregulating the discharge streams. Increasing the downstream dischargeentrains more air coming from the downstream of the furnace and preventsthis nitrogen-containing air from passing through the flames and formingNOx.

Advantageously, these regulations result in as low a nitrogen content aspossible in the flue gas discharged into upstream outlets. This contentis preferably maintained at below 10% and more preferably below 5%.

The temperature of the downstream discharge gas is generally slightlyhigher than the temperature of the flue gas discharged upstream, for thereason that the flue gas is in contact with the least hot zones of thefurnace because, in particular, close to the furnace charging point,there are normally no burners and because the covering of supernatantbatch materials absorbs a substantial portion of the energy in meltingthese batch materials.

The flue gas residence time in the furnace depends on a number ofconditions. These include, apart from the organization of the flue gasflow as indicated above, the flow rate of the flue gas produced and thevolume occupied by this flue gas within the furnace must also be added.For a given flue gas flow rate, the average residence time depends onthe available volume. The larger the volume, the longer the residencetime and, in principle, the more complete the heat transfer.

In practice, increasing the volume of the furnace has a limitedinfluence and may lead to less satisfactory economics if it is not wellcontrolled, for the following reasons. Experience shows firstly that theheat transfer to the mass to be melted and to the melt mainly takesplace through radiation. Flue gas convection contributes only less than10% of the supply, and this is very often less than 8%. Under theseconditions, increasing the flue gas residence time adds little to thisconvective supply. Moreover, increasing the volume of the furnace alsoleads to additional investment in terms of refractories and results inan additional loss of energy dissipated to the outside, whatever thequality of the furnace insulation, this loss depending on the area ofthe walls exposed to the surrounding atmosphere.

Advantageously, the discharge temperature of the flue gas is slightlyreduced as a result of its residence in the furnace. Ordinarily, in anair-fuel combustion furnace, the flue gas is at a temperature below1650° C., preferably below 1600° C. and particularly preferably below1550° C. However, in the case of an oxy-fuel combustion furnace, it isat a temperature below 1500° C., preferably below 1450° C. andparticularly preferably below 1350° C.

Moreover, the volume of the furnace also determines the velocity of theflue gas therein. It is preferable to ensure that the flue gas flowvelocity in the furnace remains moderate so as to avoid disturbing theflames. It is also necessary to prevent the fly-off of dust as the gaspasses over the batch materials, which dust would then have to beremoved before the gas passes through the heat exchangers.

From experience, it is found that the average residence time of the fluegas in an air-fuel combustion furnace is 1-3 seconds. In the case of theinvention, with an oxy-fuel combustion furnace, the mean residence timeof the flue gas lies between 10 and 40 seconds and more advantageouslybetween 15 and 30 seconds.

The positioning of the burners, or better still the distribution of theenergy supply already mentioned, is an important factor not only asregards energy consumption of the furnace but also the quality of theglass produced.

All large glass furnaces conventionally comprise two zones,corresponding to melting and to refining respectively. Beyond therefining zone, the glass continues to pass through a conditioningchannel in which the glass temperature is gradually lowered until itreaches its forming temperature. To manufacture flat glass by the“float” technique, this temperature is around 1100° C.

Usually, the refining zone is separated from the conditioning zone by aneck, which makes it possible in particular to restrict the atmosphereof one zone passing into the other. According to the invention, effortsare made to minimize the corresponding opening and consequently the flowof the atmosphere coming from the conditioning zone entering therefining zone. In all cases, the flue gas must not penetrate into theconditioning zone, otherwise dust still in suspension could be entrainedtherewith and deposited on the surface of the glass.

The ingress of gas not arising from the combustion, and in particularthe gas penetrating via the neck, is also limited as far as possible andadvantageously does not exceed 15%, and preferably is less than 10%, ofthe total volume of gas flowing through the furnace.

The distinction between melting zone and refining zone involves what isconventionally called “convection currents” in the glass. Theseconvection currents are generated by two phenomena: natural convectionand forced convection. Firstly, natural convection movements areassociated with the temperature conditions and with the powerdistribution along the furnace (called the “fire curve”). Secondly, theforced convection movements are associated with the modification of theflow caused for example by bubblers, mixers or dams. These twoconvection phenomena progress the glass, which, in the melting zone,develops forwardly flowing movements on the surface and rearwardlyflowing movements close to the bottom of the furnace. In the refiningzone, the direction of circulation is reversed.

In general, the melting zone is that requiring the greatest energysupply, and therefore that in which the overall power of the burners isgreatest. The distribution is such that this supply is not less than40%, preferably not less than 50%, of the total. It may represent up to80% of the supply, but preferably does not represent more than 70% ofthe energy delivered. The percentages in question relate to the powerdelivered by the burners that are suspended above the zone in question.

In order for the furnace to be operated as efficiently as possible, theburners must be suitably distributed along the furnace. However thisdistribution is not uniform.

It is necessary to avoid the presence of the most powerful burners closeto the flue gas outlets so as to minimize energy losses in the flue gas.However, if the glass temperatures beneath the blanket of molten batchmaterial are too low, with the risk of the glass solidifying, boostburners may be positioned close to the batch material charging points,either mounted in the walls of the furnace or in the roof. Analternative way of minimizing the energy losses in the flue gas is touse electro-booster (the glass being heated by electrodes passingthrough the bottom of the furnace). Heating by means of submergedelectrodes has the advantage of providing temperature control preciselyadjusted to the local requirements. Moreover, the efficiency of thiselectrical energy supply is much greater than in flame heating, therebyenabling it to be maintained at relatively low levels. In general whenelectrical energy is supplied, it represents no more than 10% of thetotal amount of energy developed in the furnace, and is very often lessthan 5% thereof.

The burners are located at a certain distance away from both the batchcharging points and the upstream flue gas outlets. The necessary supplyof energy in this zone therefore results firstly from the convectioncurrents within the melt, these being more intense the greater thetemperature difference between the surface covered with the batchmaterials to be melted and that further downstream in the molten glass.This energy supply secondly comes from the flue gas, which flowscountercurrently, being directed towards the outlets provided upstream.Overall, the temperature in this upstream zone is not the highest in thefurnace, but it does remain sufficient to sustain the melting.

Although the first burners are located a certain distance away from theflue gas discharge points, so as not to retard batch melting, it isnevertheless necessary to position these first burners in a zone of thefurnace in which the melt is still covered with unmelted batchmaterials. This zone preferably does not exceed one half of the lengthof the furnace, and more particularly preferably no more than one thirdthereof. This is because it is necessary, apart from melting this“blanket”, to ensure that the particles of material dispersed in themelt have melted perfectly and to ensure that the temperature has risenup to the highest point that enables not only the melting process to becompleted but also the melt to be homogenized.

Beside the positioning of the burners, the distribution of the deliveredpower is significant. The power of the burners is highest in that partof the melting zone close to the refining zone, in which part thetemperature reached is the highest.

In the refining zone, the temperature of the melt must be generallymaintained, the necessary energy supply therefore being more limited.Preferably, the burners in this zone are located in the part closest tothe melting zone. Preferably, the energy supply decreases in thedirection of advance through the refining zone.

It is necessary to be able to make variations to the furnace operatingparameters, particularly the total applied power. These variationscommanded by the nature of the batch materials, variations in output,etc. are usually of limited magnitude. To maintain the optimized energydistribution conditions as far as possible, the variationsconventionally relate essentially to the burners located furthestdownstream in the direction of advance of the glass. This particularaspect has the consequence that there are variations in flue gas volumein this zone. Consequently, to avoid modifications further upstreambecause of the break in dynamic equilibrium, it is advantageousaccording to the invention to regulate the flue gas flow rate by meansof outlets placed in the downstream zone of the furnace, as indicatedabove.

In a conventional furnace operating in air-fuel combustion mode, theburners are placed in the side walls of the furnace in such a way thatthe flames develop close to the surface of the melt. This arrangementstems partly from the limited residence time in the furnace of the fluegas, which is essentially discharged directly on that side of thefurnace facing the burner. It appears to be necessary to maximize heatexchange, including convective heat exchange, during this briefresidence time and therefore necessary to ensure that the flames arealso in contact with the surface of the melt.

In the case of operation in oxy-fuel combustion mode as in theinvention, the contribution to heat supply by convection is limited, asindicated above. It is therefore preferable to position the burners onthe side walls at a certain distance from the surface of the melt,guaranteeing suitable distribution of energy radiated directly from theflame both towards the melt and towards the roof.

Preferably, the positioning of the burners results in flames beingdeveloped in planes approximately parallel to the surface of the meltand at a distance of at least 0.25 m above this surface, and preferablyat least 0.40 m above it. This distance may be up to 1.0 m, but ispreferably less than 0.80 m.

As indicated above, the use of oxy-fuel combustion modifies the furnaceatmosphere, which contains practically no nitrogen. On the other hand,it is relatively richer in water vapour. This feature has a significanteffect on the melting behaviour. In particular, an increase in the watercontent above the melt is accompanied by an increase thereof in theglass.

The presence of a high water content so promotes degassing of the glassand makes refining easier.

One possible downside of a high water content is the formation of foamon the surface of the melt. The presence of foam is undesirable,especially as it forms an obstacle to good heat exchange. Means areknown for reducing the amount of foam if this should develop. Thesemeans, independently of the measures taken to prevent foam fromappearing, modify the surface tension of the glass, for example usingthe technique described in the publication EP 1 046 618.

Another means for minimizing the risk of foam formation in the zoneswhere it may be particularly problematic, especially in the refiningzone, consists in limiting the water vapour content by the choice offuel used in this part of the furnace.

Oxy-fuel combustion may be carried out with various types of fuelwithout losing the benefit of the abovementioned advantages. The fuelsmost often used are either natural gas or liquid fuels.

As regards the water vapour content, this is higher when a gas is usedthan when liquid fuels are used. For this reason, apart from theabovementioned energy cost questions, it may be advantageous accordingto the invention for the burners located in the refining zone to be fedwith liquid fuel. In this way, the risk of foam formation in that partof the furnace where it could be the most prejudicial is reduced.

The economic assessment of oxy-fuel combustion is based, on the onehand, on the cost of oxygen and the cost of the adapted refractoriesand, on the other hand, on the fuel savings and the savings relating tothe partial elimination of flue gas decontamination. To have a positiveoutcome, it is necessary to recover a significant portion of the heatcontained in the flue gas exiting the furnace. In practice, as in thecase of air-fuel combustion furnaces, the most efficient use consists inheating the reactants introduced into the furnace, namely oxygen, fueland possibly batch materials.

By comparison with air-fuel combustion techniques, especially thoseusing regenerators, one difficulty stems from the nature of thenecessary installations. The regenerators can receive the flue gaspractically as collected at the outlet of the furnace. The materials ofwhich the regenerators are made, especially the linings, generally madeof refractory ceramics, have no difficulty in withstanding thetemperatures of the flue gas and the dust that this gas may convey.Subsequently, the preheating of the air in the hot regenerators requiresno special precautions to be taken. In contrast, much more stringentprecautions are required when heating products used for oxy-fuelcombustion, particularly when heating oxygen. The installations in whichoxygen flows must be completely gastight, resistant to the hightemperatures and resistant to oxygen conveyed at these temperatures.

Regards the flue gas, efforts have to be made to ensure that theyentrain a minimum amount of dust. By having the flames at a certaindistance away from the surface of the melt, particularly in the zoneswhere the melt is covered with materials that have not yet melted,contributes to minimizing this entrainment. What also contributes is thefact that the burners, when they are staggered, minimize turbulence thatcould result from the impact of the gas streams emanating from burnersthat face one another.

The average velocity of the flue gas in the longitudinal direction doesnot as a general rule exceed 3 m/s and is usually less than 2 m/s. Inthe flames, this velocity is much higher, around 30 to 100 m/s, thisvelocity being normally lower than that in air-fuel combustion flames.

According to the invention, the oxygen is preheated advantageously inheat exchangers made of steel exhibiting excellent hot oxygenresistance. Heat exchangers and materials suitable for this usage aredescribed in the unpublished European patent application Ser. No.07/107,942 filed on 10 May 2007.

The hot oxygen delivered by the heat exchanger is brought to atemperature which may be at most 650° C. This value depends on theresistance that can be achieved with metal alloys having the bestcharacteristics. This limit makes it possible to guarantee a duration ofuse in relation to the type of installation in question.

In practice, for better safety it is preferable to keep the temperatureof the oxygen below 600° C.

In order for the oxygen preheating to be sufficient to improve theeconomics significantly, it is preferred to set a temperature that isnot below 350° C.

Likewise, the fuel used is advantageously preheated, whether this isnatural gas or liquid fuel. The temperatures reached by the fuel do notdepend on the resistance of the installations. However, they may dependon possible degradation of these fuels. In particular, it is necessaryto prevent them “cracking”, even partially, which would have the resultof fouling the installations. For natural gas, the preheat temperatureis advantageously below 650° C. and preferably below 550° C. For heavyfuel oils, the temperature is generally lower, not exceeding 180° C. andpreferably not exceeding 150° C.

Flue gas heat recovery is largely sufficient to allow the oxygen and thefuel to be heated to the temperatures indicted, independently of theefficiency of the heat exchange when this is carried out under theconditions given in the abovementioned patent application. It is alsopossible with the excess to preheat the batch materials or to feedboilers, whatever the usage of the steam produced.

The invention is described in certain detail below with reference to theplates of drawings in which:

FIG. 1 is a perspective schematic view of a furnace according to theinvention;

FIG. 2 illustrates schematically, in top view, the arrangements shown inFIG. 1;

FIG. 3 is an overall schematic diagram of the heat exchange circuitsused for a furnace according to the invention;

FIG. 4 is a detail of a schematic diagram relating to flow in anembodiment for preheating the oxygen; and

FIG. 5 shows a partial top view of the arrangements of the burners in afurnace according to the invention.

The furnace shown in FIG. 1 is of the type used for high-volume glassproductions, such as those serving for feeding flat glass productionunits employing the float technique. Furnaces of this type operatecontinuously and produce glass in quantities that may be up to 1000tonnes/day. To achieve this performance, furnaces must have a power ofup to 60 MW.

The furnace 1 comprises a tank placed in an enclosed chamber. Theassembly is made of refractory materials that are resistant to hightemperatures, to flue gas corrosion and to attack by the moltenmaterials. The level of the melt in the tank is shown by a dotted line2.

The furnace is fed with batch materials at one of its ends. The openingvia which these batch materials are charged is shown at 3. In practice,to make distribution over the surface of the melt easier, severalcharging points are usually provided. The output of molten glass,represented by the arrow V, is at the opposite end via a neck 4 of smallwidth compared to that of the tank. Most usually, the bottom of the neck4 is level with the bottom of the furnace.

The neck is not completely submerged in the molten glass—a gap remainsbetween the top of the neck and the surface of the glass layer. Theoperating conditions, as regards the gas streams in the furnace, areregulated in such a way that the furnace atmosphere does not passthrough the neck, so as to avoid any risk of entraining suspended dust.To ensure this operation, it is preferable to maintain a slight gasstream, indicated by the arrow A, flowing countercurrently with thestream of molten glass. Being intended only to prevent a gas stream inthe opposite direction, this stream A is kept as slight as possible. Itis important to minimize it as it is normally formed by the air presentabove the conditioning zone, not shown in FIG. 1 (but with the reference5 in FIG. 2), which comes after the neck.

Burners positioned as indicated at 6 are placed along the side walls ofthe furnace, on each side thereof, so that the flames extend practicallyover the entire width of the tank. The burners are spaced apart so as todistribute the energy supply over a major portion of the length of thismelting/refining tank.

The combustion gas F is discharged predominantly via the outlets 7located near the furnace charging zone and at a certain distance fromthe closest burners. In the representation shown (FIGS. 1 and 2), twooutlets 7 are placed symmetrically on the side walls, whereas the batchmaterials (MP) are charged along the axis of the furnace. This is apreferred embodiment, but other arrangements are also possible such as,for example, the outlet of gas in the wall 8 closing off the furnace inits upstream part. These outlets may also be distributed differently,the important point being to ensure that the flue gas flows backcountercurrently with the flow of glass V in the furnace. Whereappropriate, the discharge of the flue gas may especially take place, atleast partly, via the batch charging opening or openings.

As indicated above, according to the invention, measures are taken toensure that the enclosure of the furnace is practically impermeable tothe penetration of external air. Flow of flue gas towards the upstreamend prevents penetration on this side of the furnace. The passagewayspossibly provided on the side walls are also essentially impermeable tothe penetration of ambient air. To repel the small amount of air thatmight come from the conditioning part 5, a very limited flow of flow gasis advantageously provided towards the downstream end of the furnace.This flue gas F′ is discharged via the outlets 9.

In adjusting the amount of air flowing from the conditioning zone to therefining zone, in addition to the gas flows generated by the burners, itis also important, as indicated previously, to be able to modulate theamounts of flue gas extracted in the downstream zone of the furnace andwhich is discharged via the outlets 9.

A large-capacity glass furnace conventionally comprises two zones onecalled the melting zone and the other the refining zone. These two zonesare not delimited in FIGS. 1 and 2.

The boundary between melting and refining is not generally apparent inthe structure of the furnace. In particular, if a dam is placed on thebottom of such a furnace, this dam does not ordinarily coincide withthis boundary, even though it does play a part in determining where thisboundary is located.

The distinction between melting zone and refining zone is in all cases afunctional distinction. It corresponds to the glass circulation in thetank. This comprises a first convection current in the melting part anda second convection current in the refining part that rotates in theopposite direction to the first current. In the absence of a meanshaving a direct influence on the circulation, the position of themelting zone/refining zone boundary is determined by a number ofoperating parameters that include, in particular, the distribution ofthe energy by the burners. In FIG. 2, these two zones are shown as I andII.

As a general rule, the energy supply needed to melt the batch materialsis greater than that to keep the glass at temperature for refining.Consequently, the number of burners, and especially the power that theydeliver, is greater in the melting zone.

Although it is worthwhile supplying the maximum amount of energy formelting, and therefore right from charging the furnace with the batchmaterials, it is also necessary to avoid positioning the first burnerstoo close to the flue gas outlets 7, otherwise this very hot flue gaswould carry away with it an excessive amount of energy. The choice ofwhere the first burners are positioned is therefore the result of acompromise. The first burners are positioned in such a way that they arelocated above the supernatant materials.

Again to limit energy losses via the flue gas as indicated above, it isalso possible to vary the power of the burners according to theirposition. The first burners may especially operate at lower power thanthat of the burners positioned further downstream.

In the operating modes that have proved to be most advantageous, the“fire curve”, i.e. the temperature distribution along the furnace,firstly progresses from the upstream end as far as a central part closeto the start of the refining zone. Thereafter, the temperature varies alittle, decreasing slightly as far as the neck 4 preparing the passageinto the conditioning zone. For this reason, the downstream end of thefurnace normally has no burners.

The distribution of the burners is shown in FIG. 2 by the axis thereof.They are preferably staggered on either side of the tank so as to ensurethat the flames emanating in opposite directions do not impinge on oneanother. They are separated from one another laterally so as to bestcover the surface of the melt. In this sense too, the burners used areadvantageously of the type that develops a flame in the form of a sheetapproximately parallel to the surface of the melt. A nominal power ofeach burner depends on what burners are chosen and on the number ofburners used.

The space available on the side walls of the furnace (FIG. 5) is limitedby the presence of the metal reinforcement 12 supporting the roof of thefurnace. The beams constituting this reinforcement are closer togetherthe wider the furnace and the heavier the refractory ceramic materials.For very wide furnaces, only two flat burners 13 of the type describedin the publication WO 2004/094902 may be fitted between two successivebeams, one on each side of the furnace. These burners organize thestaged combustion, starting from a central fuel feed 14, concentric witha first oxygen feed 15, then by means of several secondary oxygen feeds16, 17 parallel to the first, these being spaced apart and lying in thesame approximately horizontal plane. These staged-combustion burners 13produce a flame that develops in a plane approximately parallel to thesurface of the melt. By their construction, these burners have a certainwidth, hence the limited number thereof between two beams.

As shown in FIG. 1, the burners open into the enclosure of the furnaceat a certain distance above the surface of the melt. This arrangement,as explained above, enables the radiated energy from the flame to becorrectly distributed and it also makes it possible, combined with theheight of the roof, to ensure correct flow of the combustion gases,especially those directed towards the main outlets 7 returning towardsthe upstream end of the furnace. Unlike in air-fuel combustion furnaces,the flue gas of which follows an essentially transverse path, in thecase of oxy-fuel combustion furnaces according to the invention the fluegas is directed along the length of the furnace and thereforetransversely to the direction of the flames that the flue gas must notdisturb. By providing space both beneath the flames and above them, theflue gas can travel without excessive turbulence prejudicial to correctdevelopment of the flames.

The flue gas discharged from the furnace is used in devices intended torecover some of the energy carried out by this gas. Although inprinciple it is possible to carry out direct heat exchange between theflue gas and the product to be preheated, for the sake of operatingoptimally, in terms of efficiency and safety, more complex heat exchangeinstallations are used.

However, in FIG. 3, for the sake of simplicity, the heat exchangeinstallations are shown overall as 18 and 19. In these installations,oxygen and/or fuel is heated before being conveyed to the burners by thelines 20, 21.

Flue gas on leaving the furnace is initially at temperatures of around1200 to 1400° C. At such temperatures, it is preferable for the flue gasto pass through a recuperator, in other words a summary heat exchangerthat enables the temperature of the flue gas to be lowered with a viewto treating it before it is discharged into the atmosphere via a chimney24. The recuperator is a system in which a fluid flows countercurrentlywith the flue gas. In its most elementary form, this comprises twoconcentric pipes. A more elaborate system is formed by a bundle of tubespassing through a chamber in which the heat-transfer fluid flows. Thetwo types may be combined.

After this recuperator, the flue gas is still at a high temperature,generally not below 700° C. except in the case of very largerecuperators being used. Before being discharged, the flue gas undergoesa decontamination treatment so as in particular to remove sulphuroxides. This removal operation is performed for example inelectrofilters. To avoid damaging these filters, the temperature must belowered further, down to about 300-400° C. One economic way of doingthis consists in diluting the flue gas with ambient air.

Lowered down to these temperatures, the mixture can still be used, forexample as a means for feeding steam-generating boilers. The steam inquestion can be used in particular to preheat liquid fuels. These areadvantageously preheated to temperatures of between 100 and 150° C.,preferably between 120 and 140° C.

The fluid heat exchanger used in the recuperators is for its part usedas indicated below with reference to FIG. 4.

In the representation shown in FIG. 3, two heat exchange installations18, 19 are placed one on each side of the furnace. The two flue gaslines are connected by a line 22. This latter line, should it benecessary to carry out maintenance or repair on one of theinstallations, allows at least some of the flue gas to be momentarilytransferred to the second installation, and the remainder may also bedischarged via the line 27 or the line 28. Similarly, a line 23 is used,if needed, to feed both sides of the furnace with heat-transfer fluid.

Lines 27 and 28 are used, where appropriate, to avoid passing the fluegas through the recuperators, sending them directly to the dischargepoint 24.

In FIG. 3, the flue gas F′ exiting downstream is not shown as pipes tothe heat exchangers. Depending on the configuration of the installation,it is also possible to connect these outlets in such a way that all theflue gas is recovered. If the “recoverable” energy is more than thatwhich can be utilized, this recovery may where appropriate be omitted.

For the reasons indicated above, it is advantageous to carry out theheat exchange in two steps. In a first “recuperator” the flue gas heatsup an intermediate fluid, for example air, nitrogen, CO₂ or anyappropriate fluid which for example circulates in a loop between thisrecuperator and a heat exchanger, or better still several heatexchangers, in which heat exchanger(s) it heats the oxygen or the fuel.An alternative situation as regards the intermediate fluid, such as air,is not to use the loop but to recover the hot air at the outlet of thesecondary heat exchangers by a boiler or another energy recovery means.

FIG. 4 illustrates this principle. The recuperator 25 receives the fluegas F and, countercurrently thereto, the fluid A, for example air. Theheated air is sent to a series of heat exchangers 26 in which it flowscountercurrently with the oxygen, which is heated before being sent tothe burners 13.

In practice, owing to the difficulty of sending hot oxygen over longlines, because of the cost of the lines or because of thermal losses,according to the invention it is advantageously proposed to preheat theoxygen close to the burners in which this oxygen will be consumed. Forthis reason, it is necessary to increase the number of heat exchangers,each of which will, depending on the situation, feed one burner or asmall number of burners located in the immediate vicinity thereof.

In FIG. 4, each burner 13 is fed by a heat exchanger 26.

The air, after preheating the oxygen, is returned to the recuperator 25or returned to the flue gas duct of the furnace, to be sent into aboiler

1. A process for melting glass in a glass melting furnace, the furnace comprising a channel-shaped melting tank, the process comprising: introducing batch materials at an upstream end, heating the furnace by burners distributed on walls along a length of the furnace, whereby at least 65% of combustion energy is produced by oxy-fuel combustion, discharging a flue gas so as to maintain dynamic sealing with respect to a surrounding atmosphere, wherein at least 40% of a total power delivered by the burners is delivered to a melting zone.
 2. The process of claim 1, wherein at most 80% of the total power delivered by the burners is delivered to the melting zone.
 3. The process of claim 1, wherein 50% to 70% of the total power delivered by the burners is delivered to the melting zone.
 4. The process of claim 1, wherein first active burners along a direction from the upstream end to the downstream end are disposed above a zone where supernatant materials are present.
 5. The process of claim 4, wherein the zone where supernatant materials are present does not exceed one half a length of the furnace.
 6. The process of claim 4, wherein the zone where supernatant materials are present does not exceed one third a length of the furnace.
 7. The process of claim 1, wherein a maximum power delivered by the burners is delivered at a boundary between the melting zone and a refining zone.
 8. The process of claim 1, wherein the burners comprise first burners, intermediate burners, and last burners disposed in order along a direction from the upstream end to the downstream end, and wherein the first burners and the last burners deliver less power than the intermediate burners.
 9. The process of claim 1, wherein the burners are disposed above a surface of molten glass at a distance of at least 0.25 m.
 10. The process of claim 1, wherein the burners are disposed above a surface of molten glass at a distance not more than 1.0 m.
 11. The process of claim 1, further comprising heating the furnace by an electrode in a charging zone of the furnace.
 12. The process of claim 11, wherein the heating by the electrode delivers no more than 10% of a total power delivered to the furnace.
 13. The process of claim 11, wherein the heating by the electrode delivers no more than 5% of a total power delivered to the furnace.
 14. The process of claim 1, comprising: discharging at least 65% of the flue gas close to the upstream end near openings through which the batch materials are introduced, and discharging at most 35% of the flue gas close to a downstream end so as to maintain dynamic sealing with respect to a surrounding atmosphere.
 15. The process of claim 1, comprising: discharging at least 75% of the flue gas close to the upstream end near openings through which the batch materials are introduced, and discharging at most 25% of the flue gas close to a downstream end so as to maintain dynamic sealing with respect to a surrounding atmosphere.
 16. The process of claim 1, comprising: discharging 100% of the flue gas close to the upstream end near openings through which the batch materials are introduced, so as to maintain dynamic sealing with respect to a surrounding atmosphere.
 17. The process of claim 1, wherein a power delivered by the burners in a refining zone decreases in a direction from the upstream end to the downstream end.
 18. The process of claim 1, wherein different fuels feed the burners in different zones.
 19. The process of claim 18, wherein a gas feeds the burners in the melting zone, and a heavy fuel oil feeds the burners in a refining zone. 